Method of treating a product pro



Patented Feb. 9, 1943 METHOD OF TREATING A PRODUCT PRO- DUCED FROM CEILUIDSE DERIVATIVE MATERIAL George Stuart Adlinz'ton, Putney Hill, London, England, assignor to Rayoid Manufacturing Company Limited, London, England No Drawing. Application June 27, 1938, Serial This invention relates to a process for covering air screws with a coating of material comprising as its essential basis nitrocellulose or cellulose acetate.

The object of the invention is to provide a method and means whereby air screws may be coated effectively with such materials without having to use complicated moulding apparatus.

According to the invention a sheet of nitrocellulose or cellulose acetate of the desired thickness is in the first place rendered plastic and deformable by a treatment as will hereinafter be described and whereby a controlled plasticizing and shrinking are assured. The treated material, which has been cut approximately to the shape of the blade of the air screw (one piece being provided for one face and another piece for the other) is laid upon the air screw so that it overlaps the edge to a predetermined amount, and it is manipulated and worked so as to make good contact and adhesion. It is generally desirable to apply to the air screw an adhesive priming coat and subsequently there may be applied a second adhesive coating to the air screw and also to one face of the sheet of the coating material.

The following is a description in more detail of a preferred manner of performing the inven tion:

A sheet of nitrocellulose of 40 mils thickness is cut approximately to the shape of one face of a blade of the air screw but somewhat larger in size to allow for eventual slight shrinkage and also to provide for a certain overlap round the edge of the blade. Such a. blank sheet of nitrocellulose is immersed in a bath of plasticizing solution having for example the composition:

Toluene 100 s., Methylated spirit (74 o. p.) 40 cos, Ethylene dichloride 80 cos.

The blank is left in this solution for an hour or more until the desired degree of plasticity is obtained.

In the meantime the surface of the blade, if made of wood, has been roughened with a men or other suitable tool to provide a better grip or anchorage for the coating to be applied. The blade will then be primed with a coating of adhesive paste which may consist, for example, of scrap cuttings of nitrocellulose dissolved in acetone, and mixed with a solution of gum elimi in acetone. This priming coat is applied over all that portion of the blade which is to be covered, and usually this will be over the whole of the blade to within six inches or so of the hub. After the priming coat on the blade has dried for In Great Britain February 15,

2Claims. (01406-198) sive coating is also applied to one side of the sheet of nitrocellulose upon its removal from the treating bath.

-The plasticized and primed sheet is then stretched over the cemented surface of the blade and brought into proper contact and adhesion at all places by rolling and/or rubbing, which operation also serves to eliminate air bubbles.

The sheet is turned over the edges of the blade so that it laps round to the other face to a distance of about three quarters of an inch from the edge and securely cemented along the edge. The blade is then allowed to dry for a period of some 12 to 48 hours at about 70 F. At the end of this time the edges of the coating sheet are trimmed oil accurately to a uniform distance of, say, half an inch to three quarters of an inch from the edge of the blade and smoothed down with a sanding machine.

The other face of the blade is likewise coated with a second sheet of nitrocellulose and this is dimensioned and applied so that it overlaps the first sheet and extends round the edge of the first coated face to a distance of about half to three quarters of an inch. It will therefore be seen that the leading and trailing edges of the air screw for about half an inch to three quarter of an inch each side of its center line will be coated with a double thicknessof nitrocellulose. This extra thickness has the advantage that it obviates the necessity of applying the brass capping which is normally used on the leading edge of the blade.

After the second sheet has dried in the course of twelve hours or more at 70 F. its edges are trimmed so that the lap over the edge is uniformly half an inch to three quarters of an inch in extent. The edge of the overlap is then rubbed down to remove any slight imperfections and the whole of the coating is lacquered over.

The the air screw is tested for balance and if adjustment is necessary this is secured by spraying Celluloid solution over the coated surface of the air screw. The solution sinks into the material ensuring homogeneity throughout.

If a metal screw is to be coated the procedure is substantially the same as has already been described with the exception that the blade is not roughened and that the priming coat preferably used is a cement made up as follows:

Per cent an hour or more a second adhesive coating is preferably put on which consists of a similar paste but excluding the gum elimi. This adhe- Pure para rubber dissolved in toluene and amyl acetate 50/50 50 Finally the coated air screws are given a finishing coat of colour and checked for balance.

It is obviously impossible to indicate every detail required for successfully coating air screws and the above particulars. are intended to serve as a general guide by which success can be attained. The following particulars are'given for the purpose of assisting in genera1 when applying the present invention:

Solutions or plasticizing baths Success was only obtained in the plasticizing of nitrocellulose and cellulose acetate in sheet form after a considerable amount of experimental work had been carried out utilizing the recognized solvents employed in the industry.

Acetone solutions, i. e. acetone mixed with alcohol and water, or acetone mixed with alcohol and petrol were found not to be satisfactory owing to their volatile nature and possibly too rapid solvent action. Furthermore there appeared to be a tendency for the treated material to become brittle due to loss of the plasticizing agent. Again, other solvents in combination with nonsolvents and high boiling solvents, such as diacetone alcohol, ethyl lactate, and glycols, were found to be unsuitable due to their slow evaporation rates, which rendered the plasticizing baths unstable and prevented effective control. The chlorinated solvents, however, yielded suitable plasticizing baths and in using ethylene dichloride, dichlor-ethyl-ether or methylene chloride, in themselves non-solvents for nitrocellulose and cellulose acetate, they are converted into solvents in the known manner on the addition of alcohol. These solutions are then rendered suitable for the present invention by the addition of either toluene, xylene or petrol.

It seems to be essential to operate with a mixture of solvents and non-solvents, constituting the piasticizing solution and to keep the evaporation rates as close as possible. The following figures compared with ether=1 will illustrate what we have in mind:

Evaporation rates Ethylene dichloride 4.7 Ethyl alcohol 8.3 Toluene 6.1

Dichlor-ethyl-ether 5.0 Ethyl alcohol 8.3 Toluene 6.1 Methylene chloride 2.0 Ethyl alcohol 8.3 Toluene 6.1

Any of the above combinations will act on nitrocellulose or cellulose acetate in sheet form and give a satisfactory result. However, since a General Nitrocellulose sheet appears preferable to cellulose acetate sheet, especially for the reason that the coated surfaces of the air screws are subjected to continual expansion, contraction and abrasion. If, due to unforeseen circumstances, when applying the plasticized material or combination of plasticizers. Camphor is the over an air screw blade an air pocket should be formed between the covering and the blade surface, this defect can be remedied by injecting Celluloid in solution, thus drawing the material into contact with the'surface. Should the coating of the blade be damaged during use it can be repaired by filling the damaged section with a paste consisting of clear nitrocellulose sheet dissolved in acetone. This paste blends with the existing covering giving an unbroken surface.

Before plasticizing, samples of both nitrocellulose and acetate sheets are tested and in the case of the former, the flash point is taken; any nitrocellulose material having a flash point lower than 180 C. is rejected. In the case of cellulose acetate, material which carbonizes at a temperature of less than 220 C. is rejected.

From tests carried out it is concluded that a nitrocellulose sheet is tougher, more elastic and practically non-hygroscopic, absorbing not more than 1% of moisture when exposed to atmospheric conditions, whereas cellulose acetate sheet will absorb up to 6% moisture.

Although ultra violet rays will cause discolouration of transparent nitrocellulose sheet, they do not cause contraction as in the case of cellulose acetate sheet. This latter material tends to contract both before and after treatment when subjected to light.

It is obvious that the number of cellulose units varies in different colour pigmented cellulose nitrate or acetate sheets and tubes, and to obtain the required expansion, it is necessary to vary the proportions of theoingredients in the plasticizlng solution. Expansion control is obtained by adjusting the proportion of alcohol in the solution, as this ingredient exerts a swelling action on the cellulose unit.

Moreover, it is also essential to adjust the proportion of toluene or other non-solvents, which retards the solvent action of, say, the thylene dichloride and alcohol plasticizing bath and a wide range of results can be obtained by making the variations, which will readily suggest themselves to those skilled in the art.

For instance in order to expand a tube three quarters of an inch diameter to one inch 8. bath consisting of:

. Parts Ethylene dichloride Ethyl alcohol 50 Toluene 80 may be employed, and if it is desired to increase the diameter of the tube still further to one and a quarter inches:

When using variations of the above ingredients satisfactory results have been obtained in the plasticizing of various of various manufactures of nitrocellulose sheets and cellulose acetate sheets of the latter those known under the trade names of Rhodoid, Bexoidl Cellon" and Ecarit" have been regularly employed. Nitrocellulose sheets do not vary a great deal in their composition, all having been manufactured under the same routine, i. e. nitrocellulose is dissolved in suitable solvents with the addition of a plasticizer extent of approximately 10%. In addition to the above plasticizers glycol phthalate is used where pliability and elasticity are essential and this plasticizer can be incorporated to the extent of 8%. All the above percentages are calculated on the weight of the original quantity of cellulose employed.

Tests for abrasion have been carried out and it has been ascertained that air screws treated with a coating in accordance with the present invention have proved more durable and efficient than any other finish at present on the market.

What I claim is:

1. The method of plasticizing a film of cellulose ester especially adapted to be applied as a covering for aircraft propellers which comprises 20 treating said film in a bath containing a chlorinated derivative selected from the group consisting of ethylene dichloride, dichlorethyl ether, and methyl chloride converted into a solvent for the cellulose ester by the addition or an aliphatic alcohol thereto and a non-solvent hydrocarbon selected from the group consisting of toluene, xylene, and petrol; the said bath also containing as a plasticizer ethylene glycol phthalate, to compensate for the loss of plasticizer removed from the cellulose ester film during treatment in the said bath.

2. The method of plasticizing a film of cellulose ester especially adapted to be applied as a covering for aircraft propellers which comprises treating said film in a. .bath containing ethylene dichloride converted into a solvent for the cellulose ester by the addition of ethyl alcohol thereto and toluene; the said bath also containing as a plasticizer ethylene glycol phthalate to compensate for the loss of plasticizer removed from the cellulose ester film during treatment in the said bath.

GEORGE STUART ADLINGTON. 

